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Apr 23, 2012 - This paper reports a new synthesis of biobased polymers by using itaconic anhydride (IAn) and lactic acid (LA) as renewable starting ma...
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Renewable Biobased Polymeric Materials: Facile Synthesis of Itaconic Anhydride-Based Copolymers with Poly(L-lactic acid) Grafts Tomoya Okuda,† Kiyoaki Ishimoto,† Hitomi Ohara,*,† and Shiro Kobayashi*,‡ †

Department of Biobased Materials Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Center for Nanomaterials and Devices, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

‡

S Supporting Information *

ABSTRACT: This paper reports a new synthesis of biobased polymers by using itaconic anhydride (IAn) and lactic acid (LA) as renewable starting materials. Poly(lactic acid) (PLA)graft copolymers were synthesized via two approaches. First, the macromonomer approach utilized IAn for Sn-catalyzed synthesis of PLA-containing macromonomers (IAn-PLA Macro). The macromonomer was radically copolymerized with n-butyl methacrylate (BMA), n-butyl acrylate (BA), methyl methacrylate (MMA), and ethyl methacrylate (EMA) to give efficiently graft copolymers (PLA-Graft copolymer (I)) with molecular weight Mn up to 1.61 × 105 having biomass content higher than 34 wt %. Second, the copolymer approach employed first IAn as comonomer for radical copolymerization with BMA, giving rise to IAn-BMA copolymer with Mn higher than 5.76 × 104. Then, Sn-catalyzed grafting of PLA onto IAn moiety of the copolymer produced PLA-Graft copolymer (II) with Mn higher than 5.88 × 104, showing biomass content ≥29 wt %. In addition, radical homopolymerization of IAn was examined to give polyIAn. By using these two approaches employing IAn as a starting reactive material, PLA-graft copolymers were obtained as “biomass-plastics”. Properties of PLA-Graft copolymers (I) were also examined, which revealed possible applications for coatings and plastics. Furthermore, the IAn-containing graft copolymers will be a convenient starting biomass polymer having reactive IAn moiety in the main chain for further grafting or various functional group-introducing reactions.

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chemicals derived from biomass.33−35 Accordingly, itaconic anhydrides (IAn) belongs to a renewable resource. In polymer chemistry, radical reactivity of IAn was examined,36−39 and in polymer synthesis IAn was utilized for radical copolymerization with methacrylate-terminated PLA macromonomer and with N-vinyl-2-pyrrolidone for the preparation of a polymeric surfactant.40,41 The present paper reports a new synthesis strategy of biobased graft copolymers starting from IAn and LA, both being typical renewable resources.1−16,33−35

INTRODUCTION Currently, biobased materials have been paid much attention due to environmental problems, typically for reducing the carbon dioxide emission. Consuming fossil fuel resources like petroleum and coal increases the net amount of carbon dioxide in the atmosphere and effects the global warming eventually. To mitigate this tendency, the concept of “carbon neutral” was created; even though the biobased materials are burned, the carbon dioxide amount is not increased, because the carbon source of biobased materials are virtually from the atmosphere via photosynthesis by plants. In the polymeric materials field, it is considered very important to perform research on polymer synthesis using renewable resources as starting materials.1 Poly(lactic acid) (PLA) is one of the most well-known and extensively studied polymers prepared from renewable resources.2−16 Synthesis of PLA has been achieved mainly via two routes; ring-opening polymerization of lactide with a variety of metal or nonmetal catalysts and direct polycondensation of lactic acid (LA) with acid catalysts.2−16 As part of investigations in this direction to conduct “green polymer chemistry”,17−29 we have recently developed a new miniemulsion system of PLA-graft copolymers for coating and elastic materials,30 and a novel oligomerization of alkyl lactates.31,32 Itaconic acid (IA) is produced in a large scale by fermentation process and regarded as one of key platform © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Materials. Itaconic anhydride (IAn, Aldrich Inc.), L-lactide (Purac Biochem bv, Gorinchem, Holland), n-butyl alcohol (BuOH) (Kanto Chemical Co.), n-butyl methacrylate (BMA), n-butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA), and tin octoate (Sn(Oct)2) (Nacalai Tesque Inc., Kyoto) were commercial reagents and used as received. A radical initiator, azobis(isobutyronitrile) (AIBN) (Nacalai Tesque Inc.) was a commercial reagent. Toluene, 1,4-dioxane (DON), chloroform, deuterochloroform (CDCl3), and other solvents and reagents were commercially available and used without further purification. Received: February 23, 2012 Revised: April 10, 2012 Published: April 23, 2012 4166

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Scheme 1. Whole Reaction Processes via Two Approaches

Model Reactions of IAn with n-Butyl Alcohol. For the 1.0:1.0 molar reaction: A mixture of IAn (1.0 g, 8.9 mmol) and BuOH (0.66 g, 8.9 mmol) with Sn(Oct)2 catalyst (0.073 g, 0.18 mmol) in 1.0 mL of toluene solvent in a 50 mL round-bottom flask with a refluxing cooler was reacted at 80 °C for 3 h under nitrogen. Then, the reaction mixture was dissolved in chloroform, washed with 1 N hydrochloric acid to remove the catalyst. The product was isolated by drying under reduced pressure to give 1.50 g of an oily product (90% yields). Furthermore, IAn is so reactive, and hence, without the catalyst the 1.0:1.0 molar reaction proceeded to give the product in close yields via similar reaction procedures. For the 1.0:3.0 molar reaction: A mixture of IAn (1.0 g, 8.9 mmol), BuOH (1.98 g, 26.7 mmol), and Sn(Oct)2 (0.066 g, 0.16 mmol) in 3.0 mL of toluene solvent in a 50 mL round-bottom flask with a refluxing cooler was reacted at 60 °C under a reduced pressure from 100 kPa to10 kPa for 45 min and reacted further at 60 °C for 2 h for giving the 1.0:2.0 adduct. The reaction mixture was dried under reduced pressure to produce 1.55 g of the product (52% yields based on the total reactants). Synthesis of Macromonomer (IAn-PLA Macro). A typical run (code 3, Table 1) is shown.4 Macromonomer having a polymerizable

group of methacryloyl-type (IAn-PLA Macro) was synthesized via one-pot, two-stage reaction. At the first stage, ring-opening polymerization of L-lactide (6.5 g, 45 mmol) by using BuOH (1.1 g, 15 mmol) initiator and Sn(Oct)2 (0.030 g, 0.075 mmol) catalyst in toluene solvent (1.5 mL) was carried out in a 50 mL flask at 110 °C for 3 h under nitrogen. The lactide was consumed quantitatively, evidenced by the 1H NMR analysis of the reaction mixture. Subsequently, at the second stage, IAn (5.04 g, 45 mmol) was added to the reaction mixture to allow the reaction of IAn with the in situformed PLA at 90 °C for 3 h. The product was then dissolved in chloroform and washed with 1N-hydrochloric acid and further washed with distilled water. The macromonomer was obtained after evaporation of chloroform to give 8.4 g (92% yields). The 1H NMR analysis of the product gave IAn-PLA Macro with m = 5.8 value and functionality of 98.2%. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA Macro with BMA, BA, MMA or EMA. A typical run (code 3, Table 2) is given as follows. A mixture of 1.8 mmol (1.0 g) of IAn-PLA Macro (m = 5.0) and 4.8 mmol (0.68 g) of BMA in 0.66 mL of toluene was reacted with AIBN initiator (5.0 mol % for the total monomer) at 70 °C for 24 h under nitrogen. The resulting polymer 4167

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was dissolved in chloroform and the solution was poured into nhexane to precipitate PLA-Graft copolymer (I), giving 1.48 g (88% yields) after drying. Synthesis of IAn-BMA Copolymer. A typical run (code 1, Table 5) is given. A mixture of IAn (0.50 g, 4.5 mmol) and BMA (1.9 g, 13.4 mmol) in 5.0 mL of DON in a two necked 100 mL round flask was subjected to copolymerize by AIBN initiator (0.03 g, 1.0 mol % for the total monomer) at 70 °C for 24 h under nitrogen. The DON solution was poured into a large amount of methanol to precipitate the product IAn-BMA copolymer, to give 2.1 g (87% yields) after drying. Synthesis of PLA-Graft Copolymer (II) via Grafting of PLA onto IAn-BMA Copolymer. A typical run (code 1, Table 6) is shown as follows. A mixture of IAn-BMA copolymer (1.0 g, 1.9 mmol of IAn unit), PLA (m = 5.0, 2.41 g, 5.6 mmol units), and Sn(Oct)2 catalyst (0.030 g, 4 mol % for IAn unit) in DON solvent (3.7 mL) was allowed to react at 90 °C for 24 h under nitrogen. Then, the reaction mixture was cooled by using a cold water bath, and the DON solution was poured slowly to diethyl ether to precipitate the polymeric materials to give 0.73 g (40% yields) after drying in vacuo. Homopolymerization of Itaconic Anhydride (IAn). To a solution of IAn (2.0 g, 17.8 mmol) in toluene (2.0 mL), AIBN (14.5 mg, 0.09 mmol) was added and the mixture was allowed to react at 70 °C with stirring for 24 h under nitrogen. White powdery precipitates were formed during the reaction. Then, 20 mL of ethyl acetate was added to the reaction mixture and the powdery material was obtained by filtration of the solution with removing the unreacted IAn to give 1.73 g (87% yields) of polyIAn. PolyIAn was soluble in methanol whereas insoluble in ethyl acetate and chloroform. Preparation of Cast Film of PLA-Graft Copolymer (I). The graft copolymer (1.0 g) obtained from BA was dissolved in 14 mL of chloroform and the solution was placed in a Tefron shale (d = 75 mm). The shale was covered with an aluminum foil and kept standing for 2 days at room temperature under atmospheric pressure, and the film was further dried in vacuo to give a cast film of the graft copolymer. Analytical Methods. 1H NMR measurements were recorded on a spectrometer ARX-type (300 MHz, Bruker Co.). CDCl3 was normally used as solvent. ESI−TOF−MS analysis was performed by using a microTOF instrument (ESI−TOF−MS) (BRUKER DALTONICS, Germany). Molecular weight of polymers was measured by a gel permeation chromatography (GPC) instrument (GL-7400 Series, GL Science Inc., Japan) with a refractive index (RI) detector using chloroform eluent at a column temperature 40 °C, in which polystyrene standards (molecular weight =2.2 × 103 − 6.5 × 105) were employed. For the polyIAn sample measurement, DMF solvent containing LiBr (10 mmol/L) was employed with the sample concentration of 10 mg/mL after membrane-filtration. Measurement conditions: column of 7.8 mm o.d. × 30 cm (TGK-GEL G3000HHR, Tosoh Co., Japan), Hitachi RI Detector (L-7490, Hitachi Hi-technologies Co.), PMMA standards, and eluent flow rate of 1.0 mL/min at 40 °C, were used. Differential scanning calorimetric (DSC) analysis was carried out using a DSC-50 (Shimadzu Co., Japan) under N2 flow (20 mL/min) with increasing the temperature at a rate of 10 °C/min with a temperature range from −90 to +200 °C. Physical properties of the film were measured on an Autograph CATY500BH (Yonekura Co., Japan) with a strain application change rate of 2 mm/s at room temperature for the film length of 35 mm and width of 5 mm.

IAn-PLA Macro, and then, the synthesis of PLA-Graft copolymer (I) is accomplished by radical copolymerization of the Macro with a vinyl monomer via reaction (3). (II) Copolymer approach employs first radical copolymerization reaction (4) between the vinylidine group of IAn and BMA to give IAn-BMA copolymer, followed by grafting reaction (5) of PLA onto the copolymer to perform the synthesis of PLA-Graft copolymers (II). I. Macromonomer Approach. To our best knowledge, IAn has not been employed for the synthesis of macromonmers so far. Therefore, we designed to prepare an IAn-PLA macromonomer by utilizing the reactive nature of IAn with ring-opening. Model Reaction. Prior to the macromonomer synthesis, an Sn-catalyzed reaction of IAn with n-butyl alcohol (BuOH) as shown in Scheme 2 was examined as a model reaction of (2), Scheme 2. Model Reaction of IAn with BuOH for Macromonomer Synthesis

where BuOH is a model of PLA. In the products of IAn-BuOH, there are three possibilities, monoesters a and b, and diester c, depending on the attacked carbonyl position by the BuO nucleophile. It was also reported previously that the IAn versus methanol reaction afforded a conjugate carboxylic acid type product (a) as major component.42 In Figure 1, parts A and B show 1H NMR spectra of IAn and of the reaction mixture (IAn:BuOH = 1.0:1.0 in the molar ratio) together with the peaks assignment. Very specific peaks due to vinylene protons of IAn are observed; Ha proton at δ 6.40 and Hb proton at δ 5.80 in part A. In addition, ringmethylene protons appear at δ 3.62. In spectrum B of the IAnBuOH product, the larger peaks at δ 6.46 and 5.83 are assigned to Ha and Hb, respectively, ascribed to structure a. The smaller peaks at δ 6.36 and 5.74 are due to Ha and Hb, respectively, of product structure b. The peaks intensity ratio was 87:13, which corresponds to the ratio of product a and product b. Very tiny peaks at δ 6.50 and 5.88 are due to free IA produced as a sideproduct. In addition, the reaction without catalyst occurred in a similar way to give similar products, whose peak intensity ratio. i.e., the product ratio, was a:b = 89:11. When the reaction of IAn and BuOH was carried out with the 1.0:3.0 molar ratio, 1H NMR spectrum of the product showed new small peaks at δ 6.32 and 5.68, in addition to the above similar major peaks (Figure 1C). The peaks were reasonably assigned to Ha and Hb of a diester species of structure c, whose content is 4%. The product ratio is a:b:c = 85%:11%:4%. Instead of BuOH, EtOH was employed with the 1.0:1.0 molar reaction, and an adduct product ratio of 89:11 was observed, indicating a close ratio to that of BuOH.

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RESULTS AND DISCUSSION Itaconic anhydride (IAn) contains two reactive groups, an acid anhydride ring group and a vinylidine group. The present study utilizes these natures, and thus, the outline is shown by five reactions (1)−(5) in Scheme 1. The outline is constituted from two approaches. (I) Macromomomer approach involves a onepot, two-stage reaction method utilizing first the BuOH−lactide reaction (1) to give PLA, followed by reaction (2)of PLA with the acid anhydride group of IAn via ring-opening to lead to 4168

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Figure 2. 1H NMR spectra of (A) PLA and (B) IAn-PLA Macro (m = 5.8).

Macro) involving ring-opening of IAn. Using 2−3 molar excess amount of IAn was preferred. Figure 2B shows 1H NMR spectrum of the product IAn-PLA Macro. Similarly to the model reaction in Scheme 2, vinylidine protons showed two peaks. Taking the results of the model reactions into account, larger peaks at δ 6.48 and 5.89 were ascribed to Ha and Hb, respectively, of the conjugate carboxylic acid type a in Scheme 2. Smaller peaks at δ 6.46 and 5.83 were assigned at Ha and Hb, respectively, of the nonconjugate type b. The ratio of a and b is 87:13. IAn-PLA Macro has the structure corresponding to type a as the main (at least, > 85% content) and hence its structure is hereafter formulated as in reaction 2. The terminal-methine peak b at δ 4.35 in Figure 2A completely disappeared after the reaction in Figure 2B, indicating a quantitative reaction to give functionality of 100%, which means the vinylidine group located quantitatively at the PLA end. In addition, −CO2H protons of the macromonomer appear very broadly at δ 8−12. Synthesis results of IAn-PLA Macro under various reaction conditions are given in Table 1. By varying the feed ratio of BuOH and L-lactide, m value can be tuned. The reaction yields were almost quantitative and functionalty of ∼100% was realized in all cases. For reference, biomass content (weight of PLA and IAn components) × 100/(total weight of the Macro) is also recorded. ESI−TOF−MS trace of IAn-PLA Macro (sample code 3 in Table 1, m = 5.8) is given in Figure 3, indicating that the Macro contains the PLA oligomers with a peak top at m = 6 in every 72 m/z interval of the repeat mass. Not only even numbered oligomers and polymers (m = 2, 4, 6, 8, 10, 12, 14 and 16) but also odd numbered ones (m = 3, 5, 7, 9, 11, 13 and 15) are formed via the Sn-catalyzed ring-opening polymerization of Llactide. These results show that the Sn-catalyzed polymerization of lactide induced the extensive occurrence of transesterification during the reaction, as discussed on the mechanistic aspects.43−45 In particular, such transesterification became noticeable after a relatively longer reaction time which was enough to reach the monomer-polymer equilibrium concen-

Figure 1. 1H NMR spectra of (A) IAn, (B) the reaction product of IAn:BuOH = 1.0:1.0, and (C) the reaction product of IAn:BuOH = 1.0:3.0.

From these results, the reaction of IAn and an alcohol in the 1.0:1.0 molar ratio produces a type adduct as the major and b type as the minor product. But c type diester adduct, a transesterification product, was not detected via the 1.0:1.0 reaction. ESI−TOF−MS (negative) analysis of the 1.0:1.0 reaction product showed the main peak at m/e 184 corresponding to the value of an adduct of IAn-BuOH (186 − H+), supporting a clean reaction of Scheme 2. Synthesis of IAn-PLA Macromonomer (IAn-PLA Macro). IAn-PLA Macro was synthesized via one-pot, two-stage reaction method. At the first stage, according to reaction (1) in Scheme 1, poly(L-lactic acid) (PLA) was prepared by the well-known procedure; Sn-catalyzed ring-opening polymerization of Llactide with using BuOH as initiator.2−16,30 The m value of PLA, degree of polymerization, can be tuned with varying the molar ratio of BuOH and L-lactide in the feed. In the case of feed ratio of BuOH: L-lactide =1.0:3.0, after 3 h at 110 °C, a small potion of the reaction mixture was taken out and the m value of the product PLA was determined by 1H NMR analysis as m = 5.8 from the integrated peak ratio of (a + b)/b as demonstrated in Figure 2A.30 At the second stage, reaction (2) was consecutively allowed to proceed by adding IAn to the reaction mixture, in which IAn was reacted with the in situ-produced PLA catalyzed by Sn(Oct)2 at 90 °C for 3 h to give a macromonomer (IAn-PLA 4169

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Table 1. Synthesis of IAn-PLA Macromonomer (IAn-PLA Macro) reaction feeda

product IAn-PLA Macro

code

BuOH:lactide:IAn (molar ratio)

catalyst (mmol)

toluene (mL)

yield (%)

m valueb

functionalityb (%)

biomass content (wt %)b

1 2 3 4 5

1.0:2.5:3.0 1.0:3.0:2.0 1.0:3.0:3.0 1.0:3.0:3.0 1.0:6.0:3.0

0.075 0.075 0.075 0.075 0.15

1.5 5.0 1.5 1.5 1.5

96 90 92 92 95

5.0 5.6 5.8 6.0 12.2

100.1 99.8 98.2 100.2 100.3

86 87 88 88 93

a Amount of BuOH was 1.1 g (15 mmol). Reaction at the first stage at 110 °C for 3 h and at the second stage at 90 °C for 3 h. bDetermined by 1H NMR.

support this speculation, in the ESI−TOF−MS trace of PLA the corresponding similar small peaks due to the dimer formation were not observed, since PLA molecules do not contain −CO2H group. Synthesis of PLA-Graft Copolymer (I). According to reaction (3) of the graft copolymer synthesis, n-butyl methacrylate (BMA) was first employed as comonomer.30 The PLA-graft copolymer (I) was produced by radical copolymerization of IAn-PLA Macro with BMA. 1H NMR spectrum of product copolymer I (code 3, Table 2) is demonstrated in Figure 4.

Figure 3. ESI−TOF−MS (negative mode) trace of IAn-PLA Macro (m = 5.8). Figure 4. 1H NMR spectrum of PLA-Graft copolymer (I) produced via radical copolymerization from combination of IAn-PLA Macro (m = 5.0) and BMA.

tration under the reaction conditions like code 3 in Table 1.46 In addition, very small ten peaks (m/e from 874 to 1522 at every 72 interval) are due to the dimer formation of the each molecule of IAn-PLA Macro during the measurement, probably because the molecule has −CO2H group to form a dimer. To

Table 2. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA Macro with Comonomer BMA copolymerization feeda code 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Macro, mmol (m value) 0.90 0.90 1.80 1.80 1.80 1.80 1.80 2.60 0.94 0.94 0.90 0.90 0.90 3.20

(5.0) (5.0) (5.0) (5.0) (5.0) (5.0) (5.0) (5.8) (12.2) (12.2) (5.0) (5.0) (5.0) (6.0)

product PLA-Graft copolymer (I)

BMA (mmol)

reaction solvent

amount of solvent (mL)

AIBNa (mol %)

yield (%)

Mnb

PDIb

Macro:BMA (molar ratio)c

biomass content (wt %)c

2.8 4.7 4.8 8.0 8.0 4.8 8.0 7.8 2.8 4.7 2.8 4.5 4.5 9.6

bulk bulk toluene toluene toluene toluene toluene toluene toluene toluene DON DON DON toluene

0 0 0.66 0.98 0.98 2.64 3.92 1.04 0.75 1.12 0.37 1.08 0.54 1.28

5.0 5.0 5.0 5.0 1.0 1.0 1.0 1.0 5.0 5.0 2.5 2.5 5.0 5.0

80 83 88 87 83 84 90 92 72 90 67 42 62 79

35 500 110 000d 35 000 28 600 45 200 36 100 29 600 71 000 60 700 54 700 11 800 14 600d 11 100 37 600

5.0 2.1d 2.0 1.9 2.3 3.2 2.2 3.8 1.7 3.1 2.2 2.2d 2.2 2.0

1.0:2.4 1.0:4.0 1.0:3.4 1.0:5.9 1.0:5.5 1.0:2.3 1.0:4.2 1.0:2.5 1.0:2.5 1.0:4.5 1.0:2.5 1.0:4.7 1.0:4.4 1.0:2.9

53 42 46 34 36 54 41 55 70 58 52 39 40 53

Copolymerization at 70 °C with AIBN initiator in bulk, in toluene or in 1,4-dioxane (DON) for 24 h under N2. Amount of AIBN for the total monomer in mol %. bDetermined by GPC; PDI (polydispersity index). cDetermined by 1H NMR. dGPC trace is given in Supporting Information.

a

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Table 3. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA macro with Comonomer BA copolymerization feeda code 1 2 3 4 5 6 7 8 a b

Macro, mmol (m value) 1.60 1.60 1.60 1.60 1.60 0.94 0.94 3.20

product PLA-Graft copolymer (I)

BA (mmol)

reaction solvent

amount of solvent (mL)

AIBNa (mol %)

yield (%)

Mnb

PDIb

Macro:BA (molar ratio)c

biomass content (wt %)c

5.3 8.9 5.3 5.3 8.9 3.1 5.2 9.6

bulk bulk toluene toluene toluene toluene toluene toluene

0 0 0.69 0.69 1.05 0.51 0.77 1.28

5.0 1.0 10.0 5.0 5.0 1.0 1.0 1.0

79 56 78 89 60 65 85 96

14 000 79 600 21,900d 40 600 25 000 72 000 102 000 161 000

2.7 3.3 3.7d 2.0 2.4 2.2 4.6 4.8

1.0:2.5 1.0:4.1 1.0:2.0 1.0:2.3 1.0:4.0 1.0:2.4 1.0:3.7 1.0:2.4

56 46 62 57 46 70 62 59

(5.8) (5.8) (5.8) (5.8) (5.8) (12.2) (12.2) (6.0)

Copolymerization at 70 °C with AIBN initiator in bulk or in toluene for 24 h under N2. Amount of AIBN for the total monomer in mol %. Determined by GPC; PDI (polydispersity index). cDetermined by 1H NMR. dGPC trace is given in the Supporting Information.

Table 4. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA Macro with Comonomers MMA and EMA copolymerization feeda

product PLA-Graft copolymer (I)

code

a

a

M(E)MA (mmol)

toluene (mL)

AIBN (mol %)

yield (%)

1 2 3 4 5 6 7 8 9 10

0.83 1.66 0.83 1.66 0.83 1.66 0.83 1.66 1.66 1.66

0.33 0.50 0.66 1.00 0.33 0.50 0.66 1.00 0.50 1.00

1.0 1.0 1.0 1.0 2.5 2.5 2.5 2.5 5.0 5.0

77 79 73 87 79 62 75 77 79 78

11 12

2.49 4.15

0.66 1.00

1.0 1.0

88 80

Mn

b

MMA 50 000 27 000 35 000 19 300 34 200 21 300 16 200 15 000 20 100 13 000 EMA 35 200 27 400

b

PDI

Macro:M(E)MA (molar ratio)c

biomass content (wt %)c

3.1 3.5 2.8 2.2 3.5 2.7 1.7 1.8 2.5 1.9

1.0:1.2 1.0:2.1 1.0:1.0 1.0:2.0 1.0:1.3 1.0:1.4 1.0:1.2 1.0:2.0 1.0:1.9 1.0:2.2

73 65 75 66 72 71 73 66 67 64

4.8 3.0

1.0:2.3 1.0:3.5

61 53

Copolymerization at 70 °C with AIBN initiator in toluene for 24 h under N2. Feed amount of 0.83 mmol of IAn-PLA Macro (m = 5.8) was used in all runs. Amount of AIBN for the total monomer in mol %. bDetermined by GPC; PDI (polydispersity index). cDetermined by 1H NMR.

a

regard all the PLA-Graft copolymers possess biomass content higher than 34 wt %, and hence, they are categorized as “biomass-plastics”.30 Further, PLA-Graft copolymer (I) synthesis via copolymerization of IAn-PLA Macro with BA as comonomer according to reaction (3) has been carried out; the results are given in Table 3. In bulk copolymerization of BA, 5.0 mol % of AIBN gave a lower molecular weight PLA-Graft copolymer (code 1), while 1.0 mol % of AIBN produced higher molecular weight copolymers (code 2). Then, the copolymerization was performed in toluene solution (codes 3−7). The macromonomer with m = 12.2 exhibited a similar reactivity to that with m = 5.8. Likewise, copolymerization of PLA-Graft Macro (m = 5.8) with MMA and EMA was performed to produce PLA-Graft copolymer (I) with feed molar ratio from 1.0:1.0 to 1.0:5.0 (Table 4). In all runs for BA, MMA, and EMA, the graft polymers having Mn = 13 000 (MMA) to 161 000 (BA) were obtained in good yields. 1 H NMR spectra of PLA-Graft copolymers of BA (code 6, m = 12.2, Table 3), MMA (code 8, m = 5.8, Table 4), and EMA (code 12, m = 5.8. Table 4) are found in the Supporting Information as parts 1−3, respectively. These spectra fully support the structures of the copolymers, the peak assignment of which are indicated in the figures.

From the integration ratio of specific peak b (δ 4.14) and peak c (δ 3.94), the Macro unit and BMA unit are contained in the ratio of 1.0:3.4, which indicates little higher radical reactivity of BMA than the Macro. In Figure 4, assignments of other signals of the copolymer are also given. Results of synthesis of PLA-Graft copolymer (I) under various conditions are given in Table 2 using BMA as comonomer. In all runs, PLA-Graft copolymers were obtained in high yields in bulk or in toluene having higher molecular weight (Mn > 2.8 × 104, up to 1.1 × 105), while in DON resulting in lower copolymer yields having lower molecular weight. Both IAn-PLA Macro and BMA are of methacryloyltype structure; the former showed little less radical copolymerization reactivity toward BMA in all reactions, judging from the feed monomer ratio and the product copolymer composition ratio. It is to be noted that all the final isolated copolymers (I) showed a unimodal peak and were not contaminated with other homo- or copolymers by GPC analysis. Two examples of GPC traces are found in Supporting Information. Values of biomass content (wt %) of PLA-Graft copolymers was calculated from the relationship; (weight of LA and IAn components) × 100/(total weight of the copolymer). According to the definition of Japan BioPlastics Association proposed in 2006, “biomass plastics” denote the plastics containing the biomass content higher than 25 wt %. In this 4171

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The macromonomer is of methacryloyl-type structure and BA is, of course, of acryloyl-type, and hence, the former showed a little more reactive in radical copolymerizability, speculating from the feed ratio and the copolymer composition ratio. In the copolymerization between the Macro and MMA, both being of methacryloyl-type, MMA is higher in copolymerizability. The biomass content of PLA-Graft copolymers (I) is all higher than 46% with the BA graft-copolymers and also higher than 64 wt % with MMA graft-copolymers. Therefore, it is to be stressed that graft copolymers of Tables 3 and 4 are all belonging to “biomass-plastics”. II. Copolymer Approach. By using IAn as comononer, it is also possible to synthesize a similar type of PLA-Graft copolymer. Synthesis of IAn-BMA Copolymer. According to radical copolymerization (4), an IAn-BMA copolymer was first prepared with varying the feed ratio. Some results of the copolymerization are given in Table 5. IAn-BMA copolymers were obtained in good yields and the molecular weight Mn reaching to 1.1 × 105 with PDI value of 1.8. The 1H NMR spectrum of product (code 1, Table 5) is shown in Figure 5A. A peak at δ 4.1−3.9 is ascribed to −OCH2 Table 5. Synthesis of IAn-BMA Copolymers via Copolymerization of IAn with BMA copolymerization feeda

Figure 5. 1H NMR spectra of (A) IAn-BMA copolymer and (B)PLAGraft copolymer (II).

product IAn-BMA copolymer

code

IAn (mmol)

BMA (mmol)

DON (mL)

yield (%)

Mnb

PDI

IAn:BMA (molar ratio)c

1 2 3

4.5 4.5 53.6

13.4 22.3 161

5.0 5.0 30.0

87 95 54d

88 800 110 000 57 600e

1.6 1.8 1.8e

1.0:3.5 1.0:5.4 1.0:2.9

b

at δ 4.36 due to the terminal methine proton of PLA (Figure 2A) disappeared completely, indicating no starting PLA remained in the isolated graft copolymer. This was also confirmed by observing a unimodal peak by GPC analysis (see Supporting Information). These graft copolymers (II) belong to biomass plastics, because of biomass content ≥29 wt %. So far, grafting reaction (5) was less effective in terms of the reaction selectivity for the grafting; under the reaction conditions the grafting at 90 °C took place in 33 mol % for the IAn unit to give PLA-G unit in the PLA-Graft copolymer (II). At a lower reaction temperature of 70 °C, only 18 mol % of IAn unit was grafted (code 2). This relatively low efficiency of the grafting is probably due to the following reason as generally observed. The grafting is a polymer−polymer reaction like reaction (5) between IAn-BMA copolymer and PLA, which is normally harder than a polymer-monomer reaction like reaction (3) between IAn-PLA Macro and a monomer. It is to be noted that IAn-BMA copolymers contain a reactive anhydride moiety in the main chain and hence will be a good starting polymer candidate for introduction of various graft chains as well as functional groups. Present graft copolymers are soluble in an organic solvent and form a film, while their detailed property study needs further examinations. Synthesis of Poly(itaconic anhydride). In relevant to the main subject of the present study to synthesize the graft copolymers by using IAn, radical homopolymerization of IAn was examined (Scheme 3). To date, polymerization of IAn has been performed mainly from the viewpoint of radical polymerizability, and product polyIAn itself was not well characterized but methylated or modified polyIAn was mentioned.36−39 The reaction of Scheme 3 proceeded homogeniously in DON, whereas precipitates were formed in toluene or in ethyl acetate. In toluene white powdery polyIAn

Copolymerization at 70 °C with AIBN initiator (amount = 1.0 mol % for the total monomer) in 1,4-dioxane (DON) for 24 h under N2. b Determined by GPC:PDI (polydispersity index). cDetermined by 1H NMR. dFor the precipitation of the product, diethyl ether was used instead of methanol. eGPC trace is given in the Supporting Information. a

in BMA unit and a peak at δ 2.8−2.6 is to −CH2C(O) group in IAn ring unit. From the integration ratio of these peaks, molar ratio in the copolymer was determined as 1.0: 3.5. Other peaks assignments are indicated in the spectrum, supporting the copolymer structure. Radical copolymerizability between IAn and BMA is comparable judging from the molar ratio value of the feed and of the product. Synthesis of PLA-Graft Copolymer (II). Grafting the PLA chain onto IAn-BMA copolymer of code 3 (Table 5) was performed by Sn-catalyzed reaction of PLA to afford PLA-Graft copolymer (II) via reaction (5). The grafting reaction results are given in Table 6. The copolymer yield was moderate, which was partly due to the difficulty of the complete separation of the starting PLA and the product graft copolymer. The molecular weight of Mn value 5.76 × 104 was little increased to 5.88 × 104 after the grafting. The 1H NMR spectrum of the copolymer (II) (code 1, Table 6) is shown in Figure 5(B). The grafting content was evaluated by a peak δ 5.16 due to methine proton in PLA-G unit and by a peak δ 2.8−2.6 due to −CH2 in IAn unit to be 33 mol % of IAn unit of the starting copolymer. Other peak assignments are given in the spectrum, reasonably supporting the graft copolymer structure. In the spectrum a quartet peak centered 4172

dx.doi.org/10.1021/ma300387j | Macromolecules 2012, 45, 4166−4174

Macromolecules

Article

Table 6. Synthesis of PLA-Graft Copolymer (II) via Grafting of PLA onto IAn-BMA Copolymers grafting reaction of PLAa

product PLA-Graft copolymer (II)

code

reaction (°C)

Sn(Oct)2 catalyst (mmol)

yield (%)

Mnb

PDIb

PLA- G:IAn:BMA (molar ratio)

biomass content (wt %)c

1 2

90 70

0.075 0.075

40 54

58 800d 59 600

1.4d 1.3

0.33:0.67:2.9 0.18:0.82:2.9

35 29

a

Grafting reaction of PLA (m = 5.0, 2.41 g, 5.6 mmol) onto IAn-BMA copolymer of code 3 in Table 5 (1.0 g, containing 1.9 mmol of IAn units, IAn:BMA = 1.0:2.9) in 3.7 mL of DON with Sn(Oct)2 catalyst for 24 h under N2. bDetermined by GPC; PDI (polydispersity index). cDetermined by 1H NMR. dGPC trace is given in the Supporting Information.

III. Properties of PLA-Graft Copolymers. PLA-Graft copolymers (I) and (II) are all soluble in an organic solvent like chloroform. As a typical example, a transparent film was prepared by casting from PLA-Graft copolymer (I) sample derived from BA (m = 6.0, code 8 of Table 3) of a chloroform solution. The sample is of high molecular weight (Mn = 161 000) having Tg value of 11.2 °C with high biomass content of 59 wt % and showed a very good elastic property as shown by the following data: Young’s modulus, 316 kgf/cm2; tensile strength, 33.7 kgf/cm2; and elongation at break, 496.1%. These results suggest that the copolymers can be applied for usages of coatings, soft films, etc.30 With using BMA as comonomer, on the other hand, the graft copolymer (I) (code 14 of Table 2), having Mn = 37 600 with biomass content of 53 wt %, showed a Tg value of 27.0 °C. The copolymer sample is very hard while very brittle as demonstrated by a value of elongation at break, 101.3%. It is understandable from the monomer structure that both the Macro and BMA have an α,α-disubstituted structure of CH2 CRR′, and moreover, the Macro contains a bulky PLA group of R = CH2C(O)PLA, and then, the resulting copolymer should have a main-chain with condensed packing, bringing about nonflexible polymeric materials. Therefore, the copolymers from BMA may find applications in hard plastic materials area requiring a tough nature which is probably accomplished via cross-linking reaction. Further, graft copolymers (I) and (II) possess a −CO2H group in the main chain, and hence, they are regarded as derivative polymers of methacrylic acid. Their applications to this direction will also be conceivable.

Scheme 3. Synthesis of Poly(itaconic anhydride)

was obtained as isolated yields of 87%. Molecular weight was relatively lower as Mn 1,400, showing Tm = 146.8 °C. A part of IAn monomer remained unreacted in DON and ethyl acetate. The synthesis results are given in Table 7. A previous paper Table 7. Synthesis of polyIAn via Radical Homopolymerization feed for homopolymerization of IAna

product polymer of polyIAn

code

IAn g (mmol)

AIBN (mmol)

solvent (mL)

yield (%)

Mnb

PDIb

1 2 3

2.0 (18) 10.0 (89) 3.0 (27)

0.09 0.90 0.27

toluene (2.0) DON (22.0) ethyl acetate (6.7)

87 28 10

1400 − −

1.2 − −

a Reaction at 70 °C for 24 h under N2 in all runs. bPDI (polydispersity index) determined by GPC.

described that bulk polymerization gave polyIAn up to 49% yields with higher molecular weight after methylation (∼2.0 × 104), whereas solution polymerization gave a lower molecular weight polymer.37 1 H NMR spectrum of polyIAn has not been shown to date, while the present polyIAn clearly exhibits a structure of vinyl type polymer from IAn, in which two methylene peaks of the ring and the main chain broadly appear centered at around δ 2.9 and δ 2.2, respectively as shown in Figure 6.

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CONCLUSION We have demonstrated for the first time that itaconic anhydride (IAn) is conveniently used as renewable monomer for the synthesis of biobased polymeric materials of poly(lactic acid) (PLA)-graft copolymers. For accomplishing the purpose, two methods, Macromonomer Approach and Copolymer Approach, were introduced. First, the methacryloyl-type polymerizable PLA macromonomer (IAn-PLA Macro) was derived from IAn and its copolymerization with BMA, BA, MMA or EMA gave PLA-Grafted copolymers (I) with biomass content higher than 34%. It is possible that the macromonomers can be copolymerized not only with these monomers but also with other various vinyl monomers for developing the PLA graft copolymers depending on the usage of the products. Second, IAn was copolymerized with a vinyl monomer to provide various copolymers like IAn-BMA copolymer. Grafting of PLA onto the copolymer gave PLA-graft copolymer (II) having biomass content higher than 29%. These two approaches employing IAn as a starting reactive material produced a variety of PLA-graft copolymers as “biomass-plastics” possessing various potential applications. Actually, a typical example copolymer demonstrated an excellent elastic nature. Further

Figure 6. 1H NMR spectrum of polyIAn in methanol-d4. Peaks with an asterisk indicate those due to ethyl acetate remaining in the sample. 4173

dx.doi.org/10.1021/ma300387j | Macromolecules 2012, 45, 4166−4174

Macromolecules

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studies on new biobased polymer synthesis as extension, on these polymer properties, and on application of the polymers are under progress in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Three 1H NMR spectra of PLA-Graft copolymers (I) produced via radical copolymerization are given for combinations: (1) IAn-PLA Macro (m = 12.2) and BA, (2) IAn-PLA Macro (m = 5.8) and MMA, and (3) IAn-PLA Macro (m = 5.8) and EMA, along with (4) five GPC traces of copolymers. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.O.); [email protected] (S.K.). Notes

The authors declare no competing financial interest.

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on April 23, 2012, with minor text errors throughout the paper. The corrected version was reposted on April 30, 2012.

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dx.doi.org/10.1021/ma300387j | Macromolecules 2012, 45, 4166−4174